Cellular polyurethanes are prepared by generating a gas during polymerization of a normally liquid (or plastic) reaction mixture. The gas causes foaming of the reaction mixture until the polymer sets or gels into a cellular structure or foam. The reaction mixture used to form such foams generally comprises an active hydrogen-containing compound, such as a polyether, a polyester, a polymer polyol or combination thereof and a polyisocyanate.
Flexible polyurethane foam is widely used in comfort cushions (furniture, bedding, automotive seating); in textiles (apparel, blankets); in industrial packaging, acoustics, filtration, and in insulation; as well as in household furnishings, sponges and the like. The versatility of polyurethane foam results not only from the nature and variety of the reaction mixture, but also from how the foam is prepared and processed. Foams ranging widely in density and hardness, in tensile and tear properties, in resistance to compression set and fatigue, in resilience and hysteresis in durability and toughness are obtained not only by the selection of the starting materials, but by the selection of the foaming process and subsequent processing.
Various processes are known for preparing flexible polyurethane foam. The processes may be broadly classified as molding processes or free rise processes.
In molding processes a liquid (or plastic) reaction mixture is injected into a vented chamber or mold and allowed to expand and completely fill the chamber. The resulting polymer is then cured. The final product retains the same size and shape of the cavity of the chamber.
In free rise processes the liquid (or plastic) reaction mixture is introduced into an open trough or on an open conveyor at ambient pressure and allowed to rise. Free rise processes may be further characterized in steps which are taken after the foam has risen. Most simply, the foam is allowed to cure and is then cut to a desired shape. Alternatively, the foam may be felted during rise, after rise or after cure. For Example, in felting a foam, a fully cured polyurethane foam is first cut into a predetermined form and then compressed and shaped while subjected to an elevated temperature. In another example a flexible, densified polyurethane foam is prepared by allowing a polyurethane foam-forming reaction mixture to rise and form a partially cured cellular material. The partially cured cellular material is then maintained for a critical time at a critical temperature while the partially cured material has a compressive force applied to permanently reduce its volume. In such a process it is known that the compressive force causes the cellular structure of a foam to collapse.
U.S. Pat. No. 3,488,300 discloses a method of controlling cell size and preventing fissures in flexible, elastomeric, cellular polyurethane. These objects are achieved by mixing reaction components under pressure. Although a pressure of from 0.5 to 60 psig is used in the mixing chamber during component blending, there is no suggestion of a pressure imposed on the mixture during polymerization. The pressure is said to be maintained only until the reaction mixture is poured onto a casting surface or into a mold. Also, the increased cell sizes are indicative of pressure release before polymerization of the mixture.